Magnetic random access memories (MRAM) have been the object of a renewed interest with the discovery of magnetic tunnel junctions having a strong magnetoresistance at ambient temperature. These MRAMs present many advantages such as speed (a few nanoseconds of duration of writing and reading), non volatility, and insensitivity to ionizing radiations. Consequently, they are increasingly replacing memory that uses more conventional technology based on the charge state of a capacitor (DRAM, SRAM, FLASH).
A conventional MRAM cell, such as the one described in U.S. Pat. No. 5,640,343, is formed from a magnetic tunnel junction comprising a first ferromagnetic layer having a fixed magnetization, a second ferromagnetic layer having a magnetization direction that can be varied during a write operation of the MRAM cell, and thin insulating layer, or tunnel barrier, between the two ferromagnetic layers. During the write operation of the MRAM cell, the magnetization of the second ferromagnetic layer can be oriented parallel or antiparallel with the one of the first ferromagnetic layer, resulting in a low or high magnetic junction resistance, respectively.
The MRAM cell can be written using a write operation based on a spin transfer torque (STT) scheme, such as described in U.S. Pat. No. 5,695,864. The STT-based write operation comprises passing a spin polarized current through the magnetic tunnel junction via a current line connected to the magnetic tunnel junction. In contrast with MRAM cells written with an external magnetic field, the spin polarized current scales inversely proportional with the surface area of the magnetic tunnel junction. MRAM cells written with the STT-based write operation, or STT-based MRAM cells, thus hold promise for high density MRAM. Moreover, STT-based MRAM cells can be written faster than when MRAM cells are written using an external magnetic field.
Most practical implementations of the STT-based MRAM cells so far involve a so-called “longitudinal” configuration wherein the spins of the spin polarized current are injected collinearly with the magnetization of the second ferromagnetic layer. This is typically achieved by using ferromagnetic materials having in-plane magnetization (magnetization in the plane of the ferromagnetic layer), or a magnetization perpendicular to plane.
In conventional STT-based MRAM cells, the injected spins of the spin polarized current are aligned substantially parallel to the orientation of the magnetization of the second ferromagnetic layer. The torque exerted by the injected spins on the magnetization of the second ferromagnetic layer is then substantially zero.
During the STT-based write operation, the writing speed is limited by the stochastic nature of switching the magnetization of the second ferromagnetic layer. This stochastic behavior is determined by the parallel orientation of the injected spins determined by the magnetization direction of the first ferromagnetic layer, or of a polarizing layer, with respect to the direction of the magnetization of the second ferromagnetic layer. Switching of the second ferromagnetic layer magnetization is triggered by thermal activation of the magnetization; i.e., when thermal fluctuation of the second ferromagnetic layer magnetization produces an initial angle between the injected spins and this magnetization of the second ferromagnetic layer. The switching speed is typically limited by a switching delay of about 10 ns for spin polarized currents in the range of 10 MA/cm2 or less than 10 ns for currents in the range of 100 MA/cm2.
In order to be able to write the memory cell at currents below 1 MA/cm2 for current pulse widths smaller than 10 ns can be obtained by inserting a perpendicularly magnetized layer, or a perpendicular polarizer, to the magnetic tunnel junction. The perpendicular polarizer generates, even at very short pulse widths, an initial angle between the orientation of the first and second ferromagnetic layer magnetization. This initial angle maximizes the initial torque and thus minimizes the critical spin polarized current needed for the switching the magnetization of the second ferromagnetic layer.
In U.S. Pat. No. 6,603,677, the magnitude of the spin polarized current is decreased by adding a spin polarizing layer or a synthetic antiferromagnetic (SAF) multilayer to the magnetic tunnel junction. Alternatively, the saturation magnetization of the second ferromagnetic layer can be decreased, or the spin polarization level of the injected electrons in the spin polarized current can be increased, for example, by providing the tunnel barrier made from MgO.
In order to obtain a suitable crystallographic texture of the first and second ferromagnetic layers adjacent to the MgO tunnel barrier, the latter layer needs to be annealed at annealing temperatures larger than 300° C., typically comprised between 340° C. and 360° C. Typical perpendicular polarizer is made of multilayers based on cobalt/platinum or cobalt/palladium or cobalt/nickel or on rare-earths/transition metals alloys. In the case the magnetic tunnel junction comprising the MgO-based tunnel barrier and the perpendicular polarizer is submitted to the annealing temperatures above, intermixing at the interfaces of the multilayered perpendicular polarizer can occur. Moreover, the rare-earth/transition alloys can be instable at these annealing temperatures.
Conventional magnetic tunnel junction manufacturing processes comprise depositing the different layers forming the magnetic tunnel junction, including the perpendicular polarizer and the MgO-based tunnel barrier, and performing the annealing of the complete magnetic tunnel junction. Consequently, proper annealing of the MgO-based tunnel barrier and good properties of the perpendicular polarizer in the same magnetic tunnel junction. It is thus not possible to obtain simultaneously a large magnetoresistance and a well defined perpendicular polarizer in the same magnetic tunnel junction.